It has become clear over the past 3 years that the membranes of Halobacterium halobium contain a family of at least 3 spectroscopically distinquishable rhodopsin-like proteins. The structure of the chromophore of these protein pigments and their use of retinal for light absorption had been observed previously only in the sensory pigments of animal eyes, i.e. in visual rhodopsins. The halobacterial rhodopsins function in photoenergy capture as well as photosensory signaling in the H. halobium cell. Two of these pigments, bacteriorhodopsin (bR) and halorhodopsin (hR), are light-driven ion pumps which energize the cell membrane, mediating light-dependent ATP synthesis and amino acid transport. A novel selection scheme that we developed to isolate ion transport mutants has made possible a genetic approach to this system. By analyzing mutants lacking both bR and hR, we have discovered a third retinal-containing pigment, named slow-rhodopsin (sR) for its relatively slow, seconds-long, photochemical reactions. Unlike bR and hR, sR apparently does not serve in energization of the cell. Rather sR appears to function as a sensory receptor mediating the color-discriminating phototaxis responses by the organism. A comparison of sR properties to those of the other halobacterial rhodopsins will contribute to our understanding of light transduction by membranes and retinal-mediated photoreactions. Our approach is to isolate H. halobium strains genetically altered in photoenergy conversion and photosensory signaling and to analyze the mutant membranes biochemically and spectroscopically. An integrated study of all 3 pigments will contribute to our understanding of retinal-mediated photochemistry and of how energy and sensory signals are transmitted across biological membranes.